Can estoppel be used as a cause of action?

In Waddams, The Law of Contracts (Toronto, Canada Law Book Limited, 1977), the following comments appear at p. 131: Some have said that the doctrine can be a shield but not a "sword", implying that it is available only to a defendant. However, it seems irrational to make enforceability depend on the chance of whether the promisee is plaintiff or defendant, and in Combe v. Combe itself Denning, L.J., cited cases where estoppel was used as part of the plaintiff's cause of action. If the reason for enforcement is the promisee's reliance, such reliance may be just as strong whether the promisee appears as plaintiff or defendant.